There are various types and designs of vena cava filters, each having particular advantages and disadvantages. Generally, it is desirable to have a filter design which is able to be deployed endoluminally into a vessel via the jugular or femoral vein by means of the well-known Seldinger technique. The filter is carried on an introducer assembly in a radially compressed condition and once positioned at the desired location it is released and allowed to expand until it engages the walls of the vessel. It is often also desirable to remove the filter after a period of treatment, in which case the filter is designed such that it can be compressed back to its radially contracted configuration and retrieved endoluminally, normally into a sheath of the retrieval assembly. The filter is typically removed via the jugular or femoral vein.
It is important for vena cava and other filters to remain reliably in position for the duration of the period in which they are left in the patient. In particular, the filter must remain in the correct orientation and also must not migrate from the site at which is it first placed. For this purpose, filters are generally provided with filter arms or legs which spring radially outwardly to press against the vessel wall. The extremities of the legs are either sharp or have specific anchor elements which pierce into the vessel wall.
An advantageous design of filter has a conical or part-conical frame provided with a plurality of radially disposed legs coupled to one another at a closed end, or hub, of the frame. Such a filter can provide reliable filtration, can be compressed to a very small diameter for ease of delivery and can minimise the amount of foreign material implanted into the patient.
A problem which can occur with filter assemblies, and which may also be experienced with other types of implantable medical device such as stents, stent grafts, and occlusion devices, is that as a result of the opening force generated by the device struts or legs and the natural movement of the vessel, the anchoring elements can become progressively further embedded into the vessel wall and eventually may pierce through the entire thickness of the vessel. This effect could be expressed as ratcheting of the filter legs and is described in further detail below.
Examples of implantable medical devices including filters can be found in US-2012/0283811, US-2009/0306703, US-2008/0027481 and US-2013/0006294.